CVE-2026-82726

Permissive Regular Expression vulnerability in ash-project ash_phoenix lets a remote client select the tenant an Ash application uses, or degrade the request, by sending a crafted Host header. AshPhoenix.Helpers.get_subdomain/2 stripped the root domain with String.replace(host, ~r/.?#{root_host}/, ""). The root host was interpolated raw, so each . became a wildcard and any metacharacter a pattern, and the replace was global and unanchored, so a match was removed from anywhere in the string. With root_host example.com, Host: foo.exampleXcom.attacker.net returned the tenant foo.attacker.net. A metacharacter-bearing or nil root host degraded the pattern or raised on every request. The comparison was also case-sensitive, so TENANT.EXAMPLE.COM and EXAMPLE.COM slipped past the root-host allowlist. conn.host comes from the client Host header. The fix matches the root host case-insensitively and only as an exact trailing suffix. This issue affects ash_phoenix: from 2.1.26 before 2.3.25.

• CWE-178
• CWE-625

• https://cna.erlef.org/cves/CVE-2026-82726.html
• https://github.com/ash-project/ash_phoenix/commit/8306f29e77526840c89f3cd244996ce7c0fa2cda
• https://github.com/ash-project/ash_phoenix/security/advisories/GHSA-rpf8-q9jh-qxrr
• https://osv.dev/vulnerability/EEF-CVE-2026-82726
• https://github.com/ash-project/ash_phoenix/security/advisories/GHSA-rpf8-q9jh-qxrr

Published

Aug 31, 2026

Last Modified

Sep 1, 2026

Status

Deferred

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